How well the new health care law succeeds in covering millions of the poorest Americans will depend largely on undecided governors of both parties, who gathered here this weekend and spoke of the challenges of weighing the law’s costs and benefits in a highly charged political atmosphere and a time of fiscal uncertainty.

The Supreme Court’s ruling last month that the states should have the choice of whether to expand their Medicaid programs has set the stage for a frenzied year and a half in which governors will have to analyze their options, devise plans, negotiate with the federal government and successfully navigate the thorny statehouse politics that often accompany any big change. Much of the law is set to take effect in 2014, when many governors will be facing re-election.

The initial reaction to the court’s ruling split along party lines. More than half a dozen Republican governors — including those of Texas and Florida, which have the nation’s largest populations of poor uninsured residents — said they would not expand their programs because Medicaid already eats up an unsustainable share of their budgets. A slightly bigger number of Democratic governors said they would move swiftly to expand coverage in their states, with the federal government pledging to pick up all the costs at first and 90 percent of them after 2020.

But as they gathered here this weekend at a meeting of the National Governors Association, most governors in both parties said that faced with a choice they did not expect to have, they needed to study how to proceed with this significant change in federal-state relations. Not all Democrats were leaping at the chance to expand their programs, and not all Republicans were ruling it out.

“This deals with hundreds of thousands of Missourians, it deals with their health care, it deals with billions of dollars, and we will be involved in a process that defines the best fit for our state and respects the sovereignty of our state and the individuality of our state,” said Gov. Jay Nixon of Missouri, a Democrat who is facing re-election this year, as well as a Republican-led legislature.

Gov. Bob McDonnell of Virginia, the chairman of the Republican Governors Association, has been one of the sharpest critics of the health care law, but he did not rule out the possibility that he might ultimately decide to expand his state’s Medicaid program. “Honestly, I don’t think it’s responsible for my state, fully, to make the decision now,” he said.

Many Republican governors who opposed the law delayed taking action on it before the court ruled. Several, including Gov. Scott Walker of Wisconsin, now say they will wait to act until after November in the hopes that Republicans will win the presidency and the Senate and dismantle it. But Mr. Walker did not wholly reject the idea of expanding Medicaid. “We’re not ruling it one way or the other,” he said on Saturday after a meeting about controlling health care costs.

Once governors make up their minds, many could face battles with their legislatures. Gov. Mark Dayton of Minnesota, a Democrat, has made expanding Medicaid a top priority, but he faces some strong resistance from the state’s Republican-controlled Legislature. And Gov. Chris Christie of New Jersey, a Republican who has expressed doubts about expanding the program, faces a Legislature controlled by Democrats, many of whom want to implement the new law.

When Rhode Island’s legislature failed to reach an agreement on setting up a health care exchange after a disagreement about how abortions would be covered, Gov. Lincoln Chafee, an independent and former Republican, decided to create one by executive order. Mr. Chafee said here that he was moving “full speed ahead” to expand the Medicaid program because “the status quo is not acceptable.”

Even governors who seemed to be leaning in one direction or another were keeping their options open. Gov. Jack Markell of Delaware, a Democrat, said he thought the expansion would be good for his state, noting that “there is a significant cost to doing nothing,” because right now poor people without coverage get care in emergency rooms, which eventually drives up the costs for people who do have insurance. But he said he was still seeking clarifications from the federal government before absolutely committing to the expansion.

Gov. Mary Fallin of Oklahoma, a Republican, said that she was skeptical that expanding Medicaid made sense, but that her state would nonetheless do a thorough study of its options. “We wanted to come here to this meeting and to visit with the other governors, certainly to listen to the experts talk about the Supreme Court ruling and the implications, especially the staff that we have that’s doing in-depth study of the Supreme Court ruling itself,” she said.

Their decisions will have ramifications for years. Richard P. Nathan, a senior fellow at the Nelson A. Rockefeller Institute of Government who has written about American federalism, called the health care law “the biggest change in state-federal relations since the enactment of Medicaid and Medicare in the Great Society.”